1. Field of the Invention
Provides a simple and reliable method and apparatus for unloading a positive displacement pump at a predetermined and adjustable level.
When the desired pressure level at the load circuit is reached, the device diverts the output from the pump back to the supply reservoir at very low pressure. When the pressure of the load circuit drops a predetermined amount, the flow is again directed to the load. The device utilizes no sliding seals under pressure and therefore it is particularly suitable for use where the fluid being pumped is abrasive as, and/or subject to drying or hardening, for example, paint. The device further provides a differential pressure between unloading and resumption of pumping, which is proportional to the pressure setting so that the relative pressure fluctuations at the load are approximately constant over the operating pressure range. The invention further provides for low friction operation.
Positive displacement pumps are extensively used in airless paint spraying. Such devices are generally mechanically driven by an electric motor or a gasoline engine. When the pump is driven by an electric motor, it is common practice to provide a switching mechanism which will turn the motor off at a predetermined load pressure and back on when the load pressure drops by a fixed amount. When a gasoline engine is used as a power source, the pump is typically turned on and off by an electrically actuated clutch between the engine and the pump.
In an electric pump of the type described above, there are disadvantages because of the frequent starting of the motor. The high starting currents required by a capacitor start motor can lead to overheating and also require very heavy supply w.+-.ring for proper performance.
In a gasoline pump of the type described above, there are disadvantages because of the complexity and cost of a drive train which requires an electrical power source on the engine, an electrical pressure sensing circuit, and an electrically actuated clutch. Rapid clutch cycling furthermore has been shown to lead to clutch overheating and excessive wear.
Many of the electric and gasoline drive machines currently available operate with a fixed pressure differential between cut-out and cut-in of the power source. If the differential is set high enough so that the machine does not cycle excessively at the high pressure settings, then the pressure variation at low pressure settings becomes unacceptable.
2. Discussion of the Prior Art
Many of the problems with machines which interrupt the drive to the pump can be solved by utilizing an unloading type of valve in the fluid circuit which diverts the flow from the load back to the source when the pressure reaches the desired level while the machine continues to operate. Unloader valves of this type are readily available and commonly used in hydraulic and water pumping applications. These devices have never found acceptance in paint pumps because they utilize high pressure sliding seals, generally U-cups or O-rings which are not compatible with the environment in a paint pump where the fluid is very abrasive and subject to drying when left unused for extended periods of time.